1. Field of the Invention
The present invention relates to the field of excimer lamps, and in particular to a method and apparatus for heat pipe cooling of an excimer lamp.
2. Background Art
Between 60 and 90 percent of the energy input in an excimer lamp is dissipated as heat. The efficiency of excimer lamps is greater when the temperature of the lamp is lower. Thus, lamp temperatures in the range of 0 to 40 degrees C. are desirable from an efficiency standpoint. However, when an excimer lamp is not cooled, the temperature of the lamp rises to values of 50 to 130 degrees C., depending on the electrical power load and the convectional cooling conditions.
One way of cooling excimer lamps is to use water. The water is usually in direct contact with one electrode of the lamp. Since in most cases this electrode has a very high potential (on the order of 10000 V), serious electrical insulation problems arise. Thus, deionized water of the highest purity is used when the high-voltage electrode is cooled. Additionally, in many applications, cooling with water has significant disadvantages due to possible leaks and problems arising when the lamp is changed. Furthermore, the water must be contained in a closed system and cooled in an external unit. The cleanliness of the water has to be monitored and insured on a continuous base. These problems can be better understood with a review of excimer lamps.
Excimer Lamps
In excimer lamps, excited diatomic molecules (excimers) emit light in the deep ultra-violet ((V)UV), the ultra-violet (UV) or the visible spectral range when the excimers decay. One form of excimer lamp is driven by a dielectric barrier discharge (DBD). In a DBD driven excimer lamp, a high voltage is applied across a gas gap which is separated from metallic electrodes by at least one dielectric barrier. Dielectric barriers include, for instance, ceramic, glass, and quartz. FIG. 1A provides an example of a typical DBD driven excimer lamp.
DBD Driven Excimer Lamps
FIG. 1A is a side view of a coaxial DBD driven excimer lamp. The lamp envelope 100 is a transparent vessel that is typically comprised of glass or quartz. In common arrangements, an inner electrode 110 is separated by a dielectric barrier 120 from the excimer gas 130 enclosed within the envelope 100 and bounded on the outside by a second electrode 140 on the outer surface of the dielectric barrier.
FIG. 1B provides an end-on view of the same coaxial DBD lamp shown in FIG. 1A. In FIG. 1B, it can be seen more clearly that the inner electrode 110 and the outer electrode 140 are circular in shape, and that the excimer gas 130 is sealed between the two electrodes. The second electrode 140 may be a mesh which allows radiation from the plasma to be emitted through the lamp envelope. The discharge from a DBD driven excimer lamp is also widely known as xe2x80x9cozonizer dischargexe2x80x9d as the utilization of DBDs in air (or oxygen) is a mature technology to produce large amounts of ozone. DBD driven excimer lamps are used to efficiently produce excimers when using rare gases, or mixtures of rare-gases and halogens as the discharge gas. The excimers emit radiation in the deep ultra-violet ((V)UV), the ultra-violet (UV), or the visible spectral range when they decay. This radiation can be used for various photo-initiated or photo-sensitized applications for solids, liquids and gases.
Typical efficiencies of DBD-driven excimer (V)UV light sources depend on the electron densities and electron energy distribution function and can be xe2x80x9ccontrolledxe2x80x9d mainly by the applied voltage frequency and shape, gas pressure, gas composition and gas gap distance. With typical arrangements, such a DBD configuration only operates in a range of 1-20% efficiency. Using steep-rising voltage pulses, efficiencies in the range of 20-40% can be obtained. Still, what makes these light sources unique is that almost all of the radiation is emitted spectrally selectively. For photo-initiated or photo-sensitized processes, the emission can be considered quasi-monochromatic. Since many photo-physical and photo-chemical processes (e.g., UV curing and bonding, lacquer hardening, polymerization, material deposition, and UV oxidation) are initiated by a specific wavelength (ideally the excimer light source will emit close to those wavelengths), these light sources can be by far more effective than high-powered light sources that usually emit into a wide spectral range.
Cooling Excimer Lamps
Excimer lamps perform more efficiently when cooled, and air cooling is typically insufficient. Thus, water is frequently used to cool excimer lamps. However, the water is usually in direct contact with one electrode of the lamp. For example, water used to cool the excimer lamp of FIGS. 1A and 1B would be in direct contact with the inner electrode 110, the second electrode 140 or both. Since in most cases this electrode has a very high potential (on the order of 10000 V), serious electrical insulation problems arise. Without sufficient insulation the danger of electrocution exists. One method of addressing this electrical insulation problem is to use deionized water of the highest purity. Pure, deionized water is significantly less conductive than non-deionized water and acts as an insulator rather than a conductor.
Another problem of cooling with water in many applications is due to possible leaks and problems arising when the lamp is changed. Furthermore, the water must be contained in a closed system and cooled in an external unit. The cleanliness of the water has to be monitored and insured on a continuous base to ensure the purity of the deionized water. Thus, water cooling is too expensive and complex of a method of increasing an excimer lamp""s efficiency for use in certain applications.
Embodiments of the present invention are directed to a method and apparatus for heat pipe cooling of an excimer lamp. In one embodiment of the present invention, a heat pipe is used to dissipate heat from an excimer lamp. Heat pipes transfer heat at a rate that is up to 1000 times higher than copper. The heat pipe is in direct contact with at least one electrode of the excimer lamp. In one embodiment, heat is transferred through the heat pipe to a cooling point that is electrically isolated from the lamp. The cooling point has essentially the same temperature as the lamp. In one embodiment, dissipation of heat from the cooling point is done by conventional means (e.g., the use of fins, the use of forced air cooling or the use of liquids).
In one embodiment, the heat pipe is on the inside of the lamp. The heat pipe consists of 3 major parts: a section where the heat is transferred from the glass of the lamp to the heat pipe, a section that has an electrical insulation strength higher than the lamp voltage and a cooling part where the heat is transferred to the environment. In another embodiment, a heat pipe is attached to the outside of an excimer lamp. The heat pipe covers only part of the lamp. In one embodiment, since the outside electrode is grounded, no electrical insulation is necessary.
In another embodiment, two heat pipes are used, one on the inside and one on the outside of an excimer lamp. This allows efficient cooling of the lamp and operation at extremely high power levels. In yet another embodiment, a heat pipe is used with a flat lamp. One electrode is covered by a flat heat pipe. In still another embodiment, a flat heat pipe is used with a flat lamp and the heat pipe has an insulation section that electrically isolates the lamp electrode from the environment.